Stephen Hawking thought a lot about black holes. But his biggest insight, the work that connected gravity to quantum mechanics, came from thinking about entropy – a concept so fundamental that it’s crucial not only to the physics of everyday life but to the meaning of time itself.
Stated most simply, entropy is a way of quantifying the amount of disorder in a system. For the most part, it is fairly intuitive. A scrambled egg has higher entropy than a whole one. A pile of bricks and wood planks has more entropy than a house in which each brick and wood plank is in its proper place.
The principle that the total entropy in a system (or in the universe) must increase over time is known as the Second Law of Thermodynamics – which, if you’ve ever tried to keep your desk clean, may be the most relatable of all the basic physical principles.
The law doesn’t mean you can’t turn a pile of bricks and planks into a house, of course, or even that it is impossible to unscramble an egg, if you take enough time and care. It just means that reducing entropy in one place requires creating more entropy somewhere else.
In the house-building example, you will end up expending energy hammering in nails and stacking up bricks. Some of that energy will manifest as heat that will radiate away into the environment, making the air around you “messier,” in some sense. Higher temperatures mean higher entropy (because particles move around more in random ways when hotter).
Your work will inevitably create enough entropy one way or another to make up for the ordered arrangement of the bricks.
So, what does this all have to do with black holes?
In the 1970s, Hawking and other physicists were playing with two seemingly unrelated concepts: the inevitable increase of entropy in the universe; and the fact that nothing can escape from black holes.
They began to ask questions. What if you take something with high entropy and throw it into a black hole? Where does that entropy go? Have you just broken the Second Law by reducing the amount of entropy in the universe? The only logical conclusion they could come to was that black holes themselves must have entropy.
The rules of thermodynamics say that to have a defined entropy, a black hole must also have a temperature – it must produce some kind of heat that can be perceived by someone outside it. Which means there has to be a way for something (high-energy particles or radiation, in this case) to come out of a black hole.
Getting into the details of how Hawking made this all work would require a fairly deep dive into some general relativity and quantum mechanics, but the short version is that thinking about entropy gave us the link between the physics of black holes and the physics of quantum particles that revolutionised how we think about both.
When the final calculations were done, it turned out black holes should radiate heat, called Hawking radiation, that looks pretty much the same as the glow from a hot poker stuck into a fire. That prediction created entirely new paradoxes that physicists are still trying to solve.
But entropy goes even deeper than that. Many physicists argue that the relentless increase of entropy is responsible for the forward passage of time itself. Most of physics works fine backward or forward, but entropy only increases one way, which might be why we can remember the past but not foresee the future. The arrow of time is determined by whatever direction makes entropy go up.
In the grand tradition of physics, every new insight creates new questions. Connecting the arrow of time to the increase of entropy brings up the thorny issue of how the universe managed to start off in a state in which the entropy was so low that it has been able to keep increasing all this time.
While we don’t yet have an answer to this question, we hope it may point us to a better understanding of how the universe began, and what fate may befall it when the entropy reaches its maximum state at its end.
In the meantime, we will each continue to be agents of chaos in our own little pockets of the universe. We can all learn from knowing you can’t get rid of your problems by throwing them in a black hole; you’ll only have a bigger mess than when you started.
Katie Mack is an astrophysicist at North Carolina State University.
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